Method of locating anomalous zones of chemical activity in a well bore

ABSTRACT

A method of locating anomalous zones of chemical activity in a well bore by measuring one or more cationic potentials and the redox (reduction-oxidation) potential of shale cuttings obtained from the well bore at different elevations during the drilling thereof, and graphically representing the different values to obtain comparisons which are indicative of the location of petroleum formations in the well bore.

United States Patent 1 Overton 1 Jan. 16, 1973 [5 METHOD OF LOCATING ANOMALOUS 3,098,198 7/1963 Salimbeni ..324/1 ZONES OF CHEMICAL ACTIVITY IN A 2,400,420 5/1946 Horvitz ..23/230 EP W L BORE 2,387,513 10/1945 Hocott ..23/230 EP X [76] Inventor: Harold L. Overton, 54.18 Whisper- OTHER UB CATIONS mg creekHoustonTex' 77017 Mounce et al., Natural Potentials in Well Logging [22] Filed: Sept. 9, 1970 Amer. Inst. of Mining and Metallurgical Engineers T h.P b.N .1626, l-6,M 1943. 21 App]. No.: 70,661 cc u 0 pp ay Primary Examiner-Gerard R. Strecker 52- us. c1. ..324/1, 23/230 51 73/153, ArwmeyPravel, Wilson and Matthews 324/13 [511 1m. (:1 ..G0lv 3/00, E21b 49/00 ABSTRACT [58] Field of SearchW'324/l 13; 23/230 Epi A method of locating anomalous zones of chemical 73/153 activity in a well bore by measuring one or more ca- 1 tionic potentials and the redox (reduction-oxidation) [56] References C'ted potential of shale cuttings obtained from the well bore UNITED STATES PATENTS at different elevations during the drilling thereof, and graphically representing the different values to obtain 2,692,755 10/1954 Nowak ..73/153 X comparisons which are indicative of the location of 86,977 3/1957 Blagg et a1. petroleum formations in the well bore. 2,374,937 5/1945 Horvitz 3,464,000 8/1969 Ower 8 Claims, 1 Drawing Figure AE(C01*")MV AE (K )Mv Rana/r farm r441 m @400 4 @500 1 i l d) f 1 k m k w. 1 K

i D m K 3 E //,/M 3" z a 0,200 k ERA/ l A ma a 019 40 0 METHOD OF LOCATING ANOMALOUS ZONES OF CHEMICAL ACTIVITY IN A WELL BORE BACKGROUND OETHE INVENTION The field of this invention is methods for locating anomalous chemical activity in a well bore.

US. Pat. Nos. 3,098,l98 and 3,182,735 disclose the measurement of the reduction-oxidation or redox potential of drilling mud, which provided limited information as to the well formations in a well bore but which information is obscured due to the presence of added clays in the drilling mud. Furthermore, the prior art failed to recognize that the redox potential readings and cationic potential readings of shale cuttings obtained at substantially the same elevations in a well were so related that graphical representations thereof were useful in locating petroleum zones in a well.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS The FIGURE is a graphical representation of the calcium ion potential, the potassium ion potential, and the redox potential of a filtrate obtained from shale cuttings at different elevations in a well bore.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the method of this invention, a filtrate is used which is obtained from either a shale cuttings slurry, or from a drilling mud having shale cuttings therewith, as will be more evident hereinafter. Measurements are made on the filtrate, using conventional potential measuring equipment so as to obtain the reading in millivolts of the cationic potential for selected cations, such as calcium ions, potassium ions and sodium ions. Also, the method of this invention includes the measurement of the reduction-oxidation or redox potential in millivolts of the same filtrate obtained from shale cuttings from successive elevations in a well bore. The readings of such potentials are graphically represented with respect to the elevations at which the readings were obtained, so that the graphical representations provide a comparison at the successive elevations in the well bore of the chemical activity within the well bore, and particularly anomalies, so as to locate zones or formations in the well which have hydrocarbons or petroleum therein.

Considering the invention more in detail, the preferred method of this invention involves the separation of shale cuttings from drilling mud, using a conventional shale shaker of a drilling rig, or other suitable apparatus. The shale cuttings are washed free of the drilling mud, preferably using distilled water. Thereafter, the washed shale cuttings are dried in any suitable oven or heating chamber at temperatures no higher than approximately 200 C. The washed cuttings are stirred continuously while baking or drying them so as to prevent glazing, and the drying is continued until all visible water is removed and the cuttings are dry as indicated by their relatively light color and lack of steam being discharged therefrom.

Thereafter, the dry cuttings are ground, preferably using a motar and pestle, or similar equipment, until relatively fine shale particles are obtained so that the fine particles pass a screen of mesh size or smaller. The fine particles are then added to an equal weight of distilled water to form a standard slurry.

The slurry is filtered, preferably using nitrogen gas to force the filtrate through the filter at about 30 p.s.i. pressure, using a No. 50 Whatman filter paper. The filtrate is then available for use in the several measurements to be hereinafter described. The temperature of the filtrate is preferably obtained prior to the measurement so that mathematical adjustments can be made in the readings of the potentials of the filtrate, if necessa- It is to be noted that the shale cuttings are obtained from different elevations, preferably successive elevations during the drilling of a well, and the elevations from which the shale cuttings are obtained are recorded so that each of the potential measurement readings, hereinafter described, is made for a particular elevation in the well bore and these are graphically represented as will be explained. The number of elevations at which determinations are made according to the present invention, and the frequency of such determinations, will depend upon the skill and judgement of the operator, but normally, a greater frequency of such determinations will occur at the first indication of an anomaly or an excursion from a normal trend line as reflected by the graphical representations, as will be more evident hereinafter.

It should be pointed out that the foregoing procedure for obtaining a filtrate for the various potential measurements as each successive elevation is preferred, but a filtrate may be obtained by filtering successive samples of drilling mud, having therewith shale cuttings, taken from different elevations in the well bore during the drilling thereof, and filtering the solids therefrom. The filtrate which is left after the filtering operation is then available for the electrical potential determina tions in the same manner as the filtrate obtained from the slurry. However, since the drilling mud has various clay or mud additives which are essentially sodium compounds, the filtrate which is obtained by the filtering of the solids from the drilling mud having the shale cuttings therewith is only satisfactory for potential measurements other than the sodium ion potential measurements.

Although the invention is applicable to the measurement of various cationic potentials, the calcium ion and potassium ion potentials are normally the ones measured and compared with the redox potential of the filtrates obtained from the same or substantially the same elevations. in the well bore. In making the potential measurements with the filtrates, the form of the apparatus used for such measurements is conventional and no specific description or illustration is believed to be required. For example, a typical apparatus would include a container for receiving the filtrate to be tested or measured, and such a filtrate would serve as the electrolyte in the testing apparatus. A reference electrode, preferably a standard calomel electrode, is provided in the electrolyte as well as an electrode for the specific ion measurement. Thus, when making the reduction-oxidation or redox potential measurement, the other electrode is platinum or another inert metal such as gold. When measuring for the calcium ion or potassium ion potential, the electrode other than the reference electrode is sensitive to the particular ion to be measured, and a membrane formed of glass or other suitable material is provided so as to be specifically sensitive to the ions to be measured. A potentiometer between the electrodes reads the electrical potential difference therebetween which is designated AE and reads in millivolts as indicated in the drawing. By way of explanation of the theory of this invention, it will be recognized that the filtrate which is produced by either of the foregoing procedures contains ions and compounds and solutions which are indicative of the original shale environment in the well. In a hydrocarbon or petroleum environment, the shale water is highly reduced and contains increased amounts of potassium ions and decreased amounts of sodium ions. Therefore, the redox potential indicates a reduced rather than an oxidized environment where hydrocarbons or petroleum are present in a particular well formation. The voltage measurements of the calcium ions, potassium ions, sodium ions or other cations, as well as the redox potential, are in accordance with the Nernst equation of the type:

A E, =A E0 (RT/nF) (a The reference A E0 is an empirical constant which must be determined from standard potassium or any other ion solutions being measured. Variations of the A E may be used to find thick shale layers which contain pressure other than hydrostatic, which is one requirement for a gas and condensate environment. A second requirement for a hydrocarbon or petroleum environment is an electrochemical barrier towards upward movement. This is the cap zone which is detected in three ways:

a. A sudden increase in the calcium ion concentration;

b. an increase in the redox potential; and

c. a decrease in the sodium ion in the shale cuttings.

Therefore, both the redox potential and the calcium ion potential, as well as the potassium ion and sodium ion potentials may be used to detect the phase boundaries which exist at the top of the various hydrocarbon or petroleum zones or formations in a well bore. Similarly, faulted zones contain a low sodium ion potential and a high calcium ion potential, which is often a chemical seal for hydrocarbons. Such a seal is found with calcium ion anomalies which are present at substantially the same elevation in a well bore as a chemically reduced anomaly, thereby indicating the presence of a hydrocarbon or petroleum environment in the well bore at that particular elevation.

Reference is now made to the FlGURE of the drawing which shows three graphical representations, labeled graph A, graph B, and graph C. The depth in feet in a well bore from which the reading relating to filtrates obtained at such depths were obtained is shown as the left-hand ordinate of the graph in the figure of the drawing.

Graph A shows the calcium ion potential readings in millivolts, with the circles or dots representing the principal readings taken, although it will be understood that a number of other readings were in fact actually taken to produce the graph A illustrated in the drawings. Graph B was formed by a plurality of readings at successive elevations in a well of the potassium ion potential in millivolts, with major points being indicated as circles or dots in the drawing. Graph C is a graphical representation of the redox potential readings in millivolts taken at successive elevations in a well bore, with the circles or dots being merely representative of some of the readings which were actually taken and utilized for the graphical representation of graph C in the drawings.

By comparing graphs A, B, and C, it can be seen that each establishes a normal trend line, with excursions or anomalies at various elevations. For example, between approximately 10,200 feet and 10,300 feet, the calcium ion concentration shows an increase, as reflected by the decreased millivolts. The potassium ion concentration, on the other hand, decreased. The redox potential also shows a variation or excursion from the normal trend line, indicating that the area or zone encountered in the well bore between 10,200 feet and 10,300 feet is a type of sand having low porosity, probably caused by the presence of lime. The water in the sand is less reduced than the water above and below as seen on graph C, further indicating the presence of sand rather than shale. Although hydrocarbons or petroleum may be present in such a sand, its low porosity is a negative factor which indicates that it probably would be difficult to remove the petroleum from the sand.

A more positive indication of petroleum or hydrocarbons is shown on the graphs A, B and C at the depth in the well bore between 10,700 feet and 10,800 feet. At that area, graph C shows a marked shift to the left from the normal trend line, indicating that the redox potential is more negative, going from an oxidizing environment to a reducing environment. Such a shift shows that the formation is more likely to be shale which gives a more reduced potential. Such a phase shift in the redox potential is indicative of a change in the phase boundary which exists at the top of a hydrocarbon zone. The increase in the calcium ion potential between 10,700 feet and 10,800 feet, and also the increase in the potassium ion potential between such elevations substantiates the reading of the redox potential and confirms that it is indeed a pressure cap such as a fault or other pressure boundary in the area between 10,700 and 10,800 feet. The calcium is increased because of the presence of the shale at such elevation rather than sand. Although the sodium ion potential is not graphically illustrated, it could be represented side by side with the graphs A, B and C, and it would typically show a decrease rather than an increase in the presence of shale formations having hydrocarbon environments.

Between 11,000 feet and 11,100 feet, the calcium ion potential increased rapidly, but the lack of change of the redox potential indicates that the calcium ion potential reading is not significant in terms of locating hydrocarbon deposits or zones. Instead, since the redox potential does not show any substantial change, the in- The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape, and materials as well as in the details of the illustrated construction may be made without departing from the spirit of the inven tion. I

lclaim: l. A method of locating anomalous zones of chemical activity in a well bore, comprising the steps of:

separating shale cuttings from drilling mud obtained at a particular elevation in a well bore; grinding the shale cuttings to free the fine particles from the coarse particles; separating the fine particles from the coarse particles by screening same with a screen at least as small as 80 mesh; forming a slurry with the fine particles passing through said screen by adding distilled water thereto; passing said slurry through a filter to produce a filtrate prior to the addition of any additive to the cuttings other than water; repeating all of the foregoing steps with shale cuttings obtained from successive elevations in the well bore; measuring the redox potential of said filtrate produced from the shale cuttings obtained at each of said elevations; also measuring the potential of at least one cation in said filtrate produced from shale cuttings obtained from each of said elevations in the well bore; and thereafter graphically representing the measurements of the redox potential and the cationic potential at the successive elevations for enabling activity which are indicative of the presence of hydrocarbons. l

2. The method set forth in claim 1, wherein: said one cation is calcium.

3. The method set forth in claim 2, including:

measuring the potassium ion potential of said filtrate produced from the shale cuttings obtained from each of said elevations in the well bore; and

also graphically representing the measurements of the potassium ion potential at the successive elevations for further substantiating and interpreting the comparative information from the other two graphs.

4.- The method set forth in claim 1, wherein:

said filtrate is produced by filtering out the solids from drilling mud.

, 5. The method set forth in claim 11, wherein said filtrate is produced by the additional steps of:

washing the cuttings free of drilling mud; and

thereafter drying the cuttings before grinding same.

6. The method set forth in claim 5, wherein:

the cuttings are dried at a temperature no higher than 200C; and

the dried cuttings are ground to about mesh size or smaller.

7. The method set forth in claim 5, wherein: said one cation is calcium.

8. The method set forth in claim 7, including:

measuring the potassium ion potential of the filtrate from the shale cuttings from successive elevations in a well bore; and

also graphically representing the measurements of the potassium ion potential at the successive elevations for further substantiating and interpreting the comparative information from the other two graphs. 

2. The method set forth in claim 1, wherein: said one cation is calcium.
 3. The method set forth in claim 2, including: measuring the potassium ion potential of said filtrate produced from the shale cuttings obtained from each of said elevations in the well bore; and also graphically representing the measurements of the potassium ion potential at the successive elevations for further substantiating and interpreting the comparative information from the other two graphs.
 4. The method set forth in claim 1, wherein: said filtrate is produced by filtering out the solids from drilling mud.
 5. The method set forth in claim 1, wherein said filtrate is produced by the additional steps of: washing the cuttings free of drilling mud; and thereafter drying the cuttings before grinding same.
 6. The method set forth in claim 5, wherein: the cuttings are dried at a temperature no higher than 200*C; and the dried cuttings are ground to about 80 mesh size or smaller.
 7. The method set forth in claim 5, wherein: said one cation is calcium.
 8. The method set forth in claim 7, including: measuring the potassium ion potential of the filtrate from the shale cuttings from successive elevations in a well bore; and also graphically representing the measurements of the potassium ion potential at the successive elevations for further substantiating and interpreting the comparative information from the other two graphs. 